Burner controllers such as are used in furnaces and boilers must detect the presence of flame within the combustion chamber. There are a number of different flame characteristics on which the operation of flame detectors are based. One class detects the flickering infrared (IR) radiation generated by the flame. Another class senses the presence of the ionized particles which a flame produces. A third type senses the ultraviolet radiation which an active flame produces. It is possible to imagine situations where ionized particles or IR flicker may be present without an active flame. Many experts believe that ultraviolet sensing is the most reliable of these sensors since only an active flame produces enough heat to generate ultraviolet (UV) radiation. The present invention is a process for producing an improved UV flame detector.
A number of different types of ultraviolet radiation detectors have been used. Historically, the first of these were photomultiplier tubes. These have the disadvantages of requiring relatively high operating voltage, having a relatively small signal current, and having a relatively limited lifetime. There have therefore been attempts to replace these tube detectors with solid state devices.
A number of different solid state UV radiation detectors have been developed and used over the last few decades. One such type uses a cadmium sulfide material as the active element. These also require relatively high voltages and have relatively small signal currents. More recent devices using a UV-enhanced silicon semiconductor material as the active element have substantial sensitivity in the visible light spectrum, and hence require a filter which attenuates the visible light component of the radiation. These silicon devices have relatively high resistance when exposed to UV radiation. As a general principle, sensors having relatively low internal resistance are preferred, other things like sensitivity and longevity being equal. The lower the internal resistance, the higher the signal current for a given bias voltage. Higher signal current reduces the effect of noise and interference on the signal, allowing for simpler and cheaper leads and less complex amplifiers.
Where solid state sensors are concerned, the general procedure is now to create the layer of photosensitive material on an appropriate substrate. Then the metallization or electrical contacts are applied to the photosensitive layer. Frequently, this is in the form of photolithographic fingers which interleave on the photosensitive layer. A protective layer may then be applied to the layers. Then the substrate is cut into individual sensor elements and the individual sensor elements packaged.
It is now known that some gallium nitride-based detectors have superior sensitivity to UV radiation. For example, U.S. Pat. No. 5,278,435 discloses a semiconductor device having an active layer comprising gallium nitride (GaN) material and which exhibits linear response to UV radiation.
The MOCVD (metal organic chemical vapor deposition) process is another technical factor relating to this invention. The MOCVD process has for many years been standard for producing the active elements for various types of electronic devices, particularly opto-electronic devices. The MOCVD process allows one to deposit layers of metallic or semiconductor compounds on a heated substrate placed within a deposition chamber. It involves entraining an organic compound of one or more metals in a first hydrogen stream. A compound of nitrogen or other Group III element is entrained in a second hydrogen stream. The two hydrogen streams with their entrained materials are introduced into and flow through the deposition chamber, where the metallic and Group III materials deposit themselves on the heated substrate. By properly selecting the entrained elements and their concentrations (partial pressures); the temperature, pressure, duration, and flow rate parameters of the process; and the types of layers created, it is possible to make a variety of solid state semiconductor devices, including GaN devices which are sensitive to UV radiation.
The following description provides information for operating a MOCVD process for manufacturing a GaN UV radiation detector. We assume the reader is familiar with Organometallic Vapor-Phase Epitaxy: Theory and Practice, Gerald Stringfellow, Academic Press, Inc. 1989, which is a standard reference work on the MOCVD process.